Electronic hearing aids and methods are discussed in coassigned U.S. Pat. No. 4,548,082 which is incorporated herein by reference as an example of an electroacoustic system in which the present invention can be used.
Without limiting the scope of the present invention in hearing aids, public address systems and electroacoustic systems in general, the background of the invention is described specifically in connection with its application to hearing aids.
A person's ability to hear speech and other sounds well enough to understand them is clearly important in employment and many other daily life activities. Improvements in hearing aids which are intended to compensate or ameliorate hearing deficiencies of hearing impaired persons are consequently important not only to these persons but also to the community at large.
Unfortunately, presently available hearing aids are often subject to a feedback phenomenon which produces distortion, ringing, and squealing. If the hearing aid can be adjusted by the hearing impaired person to reduce the feedback, the volume delivered is usually reduced also. Not only is the person enmeshed in a dilemma of choosing between feedback and reduced volume, but also the adjustment process itself is one more inconvenience in that person's life. Not surprisingly, many users of hearing aids leave the aid misadjusted or simply put the hearing aid aside because of the inconvenience and lack of satisfactory operation with which they must cope.
Conventionally, a microphone in the hearing aid generates an electrical output from external sounds. An amplifying circuit in the aid provides a filtered version of the electrical output corresponding to the sounds picked up by the microphone. The filtering can be due to an inherent characteristic of the amplifying circuit or may be deliberately introduced. The amplified and filtered output of the hearing aid is fed to an electrically driven "receiver" for emitting sound into the ear of the user of the hearing aid. (In the hearing aid field, a receiver is the name of an electronic element analogous to a loudspeaker or other electroacoustic transducer.) Some of the sound emitted by the receiver returns to the microphone to add a feedback contribution to the electrical output of the microphone. The feedback is amplified by the hearing aid, and ringing or squealing often arise in an endlessly circular feedback process.